By definition, brazing is a process for joining similar or dissimilar metals, i.e., at least two base metals, using a filler metal that typically includes a base of copper combined with silver, nickel, zinc or phosphorus. More specifically, the assembly is heated to flow and distribute the filler metal between two or more close-fitting parts by capillary action. To achieve this, the filler metal is brought slightly above its melting (liquidus) temperature while protected by a suitable atmosphere, usually a flux. The melted brazing alloy in contact with the base metals then flows over the base metals (known as wetting) and into the joint by capillary attraction, and is then cooled to join the base metals together. Brazing is distinguished from soldering by the temperature used to melt the filler metal, which is above 450° C. (842° F.). Brazing differs from welding in that brazing does not appreciably melt the base metals, therefore brazing temperatures are lower than the melting points of the base metals. For the same reason, brazing is a superior choice in joining dissimilar metals. Brazed joints are strong, e.g., a properly-made joint (like a welded joint) will in many cases be as strong as, or stronger than, the joined base metals.
Brazing of copper and copper alloys is most often performed using phosphorus-copper brazing alloys as the filler metal, also known as phos-copper alloys, classified as BCuP 1-7 by the American Welding Society (AWS). Silver may also be added to the copper and phosphorus to accomplish special features for a wide variety of applications, and these alloys are generally known as silver-phos-copper alloys. The phos-copper and silver-phos-copper alloys are generally used for brazing together base metals of copper and the copper alloy group of metals, namely, alloys of copper with zinc, gold, tin, silver and/or antimony. Although silver and gold metals are readily brazed with AWS brazing alloys BCuP 1-7, these brazing alloys are seldom used because they are a poor color match for silver and gold.
The brazing alloy is used in the form of wire, rod, strip, powder, paste, and preform shapes, which are placed between or adjacent to the surfaces of the two parts to be joined. Depending on the application, the filler material can be pre-placed at the desired location or applied during the heating cycle. For manual or torch brazing, wire and rod forms are generally used as they are the easiest to apply while heating. High skill is often required for this labor intensive process. In the case of furnace brazing, the brazing alloy is typically placed beforehand since the process is usually highly automated.
To obtain high-quality brazed joints, parts must be closely fitted, and the base metals should be clean and free of oxides. A flux may be required to prevent oxides from forming while the metal is heated. The flux also serves the purpose of cleaning any contamination left on the brazing surfaces. Flux can be applied in any number of forms including flux paste, liquid, powder or pre-made brazing pastes that combine flux with filler metal powder. Flux can also be applied using brazing rods with a coating of flux, or a flux core. In either case, the flux first flows into the joint when applied to the heated joint and is subsequently displaced by the molten filler metal entering the joint. Phos-copper and silver-phos-copper brazing alloys can be self-fluxing when joining copper to copper, and more specifically, the phosphorus addition in the brazing alloy acts as a flux for joining copper base metals. Phosphorus lowers the brazing temperature in addition to acting as the flux and wetting and removing oxides from the surfaces being joined.
The smaller the parts to be joined, the more difficult it is for a precise joint to be formed. The brazing alloy must be completely or nearly completely molten to flow under capillary attraction, and when completely molten, the brazing alloy may flow like a thin liquid and may run onto surfaces outside the joint where the brazing alloy is not needed or wanted. Great skill and effort can be required when brazing with BCuP 1-7 alloys. The parts to be joined must be arranged carefully, and brazing alloy preforms must be made to an exacting size and required volume of metal. Brazing of small, delicate pieces of jewelry, for example, require high precision that is difficult with a brazing operation. Additionally, adding brazing flux to an assembly takes care, and the brazing process must be carefully monitored for parts that may move due to melting of the brazing flux. If a part moves during brazing, the process must be stopped to make corrections to the position. In industrial uses where joints are being formed in place, the location of the parts to be brazed may present difficulty in accurately placing the filler metal while applying the torch heat.
There is thus a need for a way to form joints that requires less skill and precision by the operator and that can be used with small parts and with base metals of copper, gold and/or silver.